I started sailing as a kid and began competitive racing in college. I continued racing on and off through several boats till about 2000. For that whole time 'cruising speed' was just how fast the boat happened to be going when not racing.
The next few years were spent battling cancer which I beat, but left me much less agile than before. I decided solo sailing was too difficult, and life was too short so I bought a Donzi to feel more alive again, yes, a mid-life crisis for sure. 'Cruising speed' for me then was a relaxed 50-55 kts with top speed well over 80.
I came to my senses a few years later and on a whim, I bought my present Fales 30. 'Cruising speed' is now 6.75 kts at 70% power. Top speed is 7 kts. and the fuel burn is negligible. This has proven to be too slow for the way I now find myself boating, mostly day cruising, where i am very limited in same-day destinations. I am searching for something small, economical and easily handled solo but, air conditioned with accomodations for the occasional overnight or weekend.
I am investingating the Mainship 30 Rumrunner Sedan. Looks like with the large Yammy, an easy 'cruising speed' of 17-18 kts and top of 26-27 kts is available at a relatively modest purchase price and fuel burn. This will almost triple my effective day-cruising range and greatly expand my choice of destinations. Also, shouldn't be too hard on the wallet, unlike the damage done by that scream-and-fly Donzi . . . which I do really miss sometimes.
So I guess my personal definition of 'cruising speed' really depends mostly on when you ask me, and what is waiting for me back in my slip.